There is a continuous and expanding need for rapid, highly specific methods of detecting and quantifying chemical, biochemical and biological substances as analytes in research and diagnostic mixtures. Of particular value are methods for measuring small quantities of nucleic acids, peptides, pharmaceuticals, metabolites, microorganisms and other materials of diagnostic value. Examples of such materials include small molecular bioactive materials (e.g., narcotics and poisons, drugs administered for therapeutic purposes, hormones), pathogenic microorganisms and viruses, antibodies, and enzymes and nucleic acids, particularly those implicated in disease states.
The presence of a particular analyte can often be determined by binding methods that exploit the high degree of specificity, which characterizes many biochemical and biological systems. Frequently used methods are based on, for example, antigen-antibody systems, nucleic acid hybridization techniques, and protein-ligand systems. In these methods, the existence of a complex of diagnostic value is typically indicated by the presence or absence of an observable xe2x80x9clabelxe2x80x9d which has been attached to one or more of the interacting materials. The specific labeling method chosen often dictates the usefulness and versatility of a particular system for detecting an analyte of interest. Preferred labels are inexpensive, safe, and capable of being attached efficiently to a wide variety of chemical, biochemical, and biological materials without significantly altering the important binding characteristics of those materials. The label should give a highly characteristic signal, and should be rarely, and preferably never, found in nature. The label should be stable and detectable in aqueous systems over periods of time ranging up to months. Detection of the label is preferably rapid, sensitive, and reproducible without the need for expensive, specialized facilities or the need for special precautions to protect personnel. Quantification of the label is preferably relatively independent of variables such as temperature and the composition of the mixture to be assayed.
A wide variety of labels have been developed, each with particular advantages and disadvantages. For example, radioactive labels are quite versatile, and can be detected at very low concentrations, such labels are, however, expensive, hazardous, and their use requires sophisticated equipment and trained personnel. Thus, there is wide interest in non-radioactive labels, particularly in labels that are observable by spectrophotometric, spin resonance, and luminescence techniques, and reactive materials, such as enzymes that produce such molecules.
Labels that are detectable using fluorescence spectroscopy are of particular interest, because of the large number of such labels that are known in the art. Moreover, the literature is replete with syntheses of fluorescent labels that are derivatized to allow their facile attachment to other molecules, and many such fluorescent labels are commercially available.
In addition to being directly detected, many fluorescent labels operate to quench the fluorescence of an adjacent second fluorescent label. Because of its dependence on the distance and the magnitude of the interaction between the quencher and the fluorophore, the quenching of a fluorescent species provides a sensitive probe of molecular conformation and binding, or other, interactions. An excellent example of the use of fluorescent reporter quencher pairs is found in the detection and analysis of nucleic acids.
An alternative detection scheme, which is theoretically more sensitive than autoradiography, is time-resolved fluorimetry. According to this method, a chelated lanthanide metal with a long radiative lifetime is attached to a molecule of interest. Pulsed excitation combined with a gated detection system allows for effective discrimination against short-lived background emission. For example, using this approach, the detection and quantification of DNA hybrids via an europium-labeled antibody has been demonstrated (Syvanen et al., Nucleic Acids Research 14: 1017-1028 (1986)). In addition, biotinylated DNA was measured in microtiter wells using Eu-labeled strepavidin (Dahlen, Anal. Biochem, 164: 78-83 (1982)). A disadvantage, however, of these types of assays is that the label must be washed from the probe and its fluorescence developed in an enhancement solution. A further drawback has been the fact that the fluorescence produced has only been in the nanosecond (ns) range, a generally unacceptably short period for adequate detection of the labeled molecules and for discrimination from background fluorescence.
In view of the predictable practical advantages it has been generally desired that the lanthanide chelates employed should exhibit a delayed fluorescence with decay times of more than 10 xcexcs. The fluorescence of many of the known fluorescent chelates tends to be inhibited by water. As water is generally present in an assay, particularly an immunoassay system, lanthanide complexes that undergo inhibition of fluorescence in the presence of water are viewed as somewhat unfavorable or impractical for many applications. Moreover, the short fluorescence decay times is considered a disadvantage of these compounds. This inhibition is due to the affinity of the lanthanide ions for coordinating water molecules. When the lanthanide ion has coordinated water molecules, the absorbed light energy (excitation energy) is transferred from the complex to the solvent rather than being emitted as fluorescence.
Thus, lanthanide chelates, particularly coordinatively saturated chelates having excellent fluorescence properties are highly desirable. In the alternative, coordinatively unsaturated lanthanide chelates that exhibit acceptable fluorescence in the presence of water are also advantageous. Such chelates that are derivatized to allow their conjugation to one or more components of an assay, find use in a range of different assay formats. The present invention provides these and other such compounds and assays using these compounds.
Luminescent (including fluorescent and phosphorescent) markers find a wide variety of applications in science, medicine and engineering. In many situations, these markers provide competitive replacements for radiolabels, chromogens, radiation-dense dyes, etc. Moreover, improvements in fluorimetric instrumentation have increased attainable sensitivities and permitted quantitative analysis.
Lanthanide chelates in combination with time-resolved fluorescent spectroscopy is a generally accepted immunochemical tool. Presently preferred lanthanide ions include, Dy3+, Sm3+, Tb3+, Er3+ and Eu3+, Nd3+, Yb3+. Other lanthanide ions, such as La3+, Gd3+ and Lu3+ are useful, but generally less preferred.
The present invention provides lanthanide complexes that are extremely luminescent and possess many features desired for fluorescent markers and probes of use in fluorescent assay systems. Among these advantages are: 1) ligands acting as both chelators and chromophore/energy transfer devices; 2) very high quantum yields of lanthanide ion fluorescence of the present complexes in water without external augmentation, such as by micelles or fluoride; 3) high stability and solubility of these complexes in water; 4) an extremely easy synthesis that employs inexpensive starting materials; and 5) facile access to many derivatives for linking these luminescent probes to, for example, an immunoreactive agent or solid support (e.g., polymer).
The present invention provides a new class of lanthanide-complexing ligands that incorporate salicylamide moieties within their structures and luminescent metal complexes of these ligands. The compounds of the invention include salicylylamide-based bidentate, tetradentate and other higher polydentate ligands. The compounds of the invention are easily prepared in good yields.
Thus, in a first aspect, the present invention provides a luminescent lanthanide metal chelate comprising a metal ion of the lanthanide series and a complexing agent comprising at least one salicylamidyl moiety.
In a second aspect, the invention provides a compound having a structure according to Formula I: 
In Formula I, R1 and R2 are members independently selected from the group consisting of alkyl, substituted alkyl, halogen and xe2x80x94OR6, wherein R6 is a member selected from the group consisting of H, alkyl, substituted alkyl groups and a single negative charge. R4, R5, R7, R10 and R20 are members independently selected from the group consisting of H, alkyl and substituted alkyl groups. R3, R8 and R9 are members independently selected from the group consisting of alkyl and substituted alkyl groups. R11, R12, R13, R21, R22 and R23 are members independently selected from alkyl, substituted alkyl, H, xe2x80x94NR14R15, xe2x80x94NO2, xe2x80x94OR16, xe2x80x94COOR17, wherein, R14, R15, R16 and R17 are members independently selected from the group consisting of H, alkyl and substituted alkyl, wherein R12 can optionally form a ring with R11, R13 or both, and R22 can optionally form a ring with R21, R23 or both. The rings are members independently selected from the group of ring systems consisting of cyclic alkyl, substituted cyclic alkyl, aryl, substituted aryl, heteroaryl, substituted beteroaryl, heterocyclyl and saturated heterocyclyl ring systems. Q1 is xe2x80x94ORxcx9cand Q2 is xe2x80x94OR19, wherein R18 and R19 are members independently selected from H, an enzymatically labile group, a hydrolytically labile group and a single negative charge. The letters a and z are independently selected from the group consisting of 0 and 1, with the proviso that when a is 0, N1xe2x80x2 is covalently attached directly to carbonyl 1xe2x80x2, and when z is 0, N2xe2x80x2 is covalently attached directly to carbonyl group 2xe2x80x2.